1. Field of the Invention
The present invention relates to a thermal transfer printing sheet useful for transferring images, letters, symbols or the like on both surfaces of an image-receiving material by a thermal transfer printing process, and to a process of double-side transfer printing using the same.
2. Related Art
Heretofore, a thermal transfer printing process has been used to form images with gradation, or monotone letters, symbols or the like on an image-receiving material. The heat-sensitive sublimation transfer printing process or the hot-melt transfer printing process is extensively employed as the thermal transfer printing process.
The heat-sensitive sublimation transfer printing process is as follows: a thermal transfer printing sheet comprising a substrate sheet, and a sublimable dye layer provided thereon, containing a binder resin, and, as a coloring material, a sublimable dye which is dissolved or dispersed in the binder resin is superposed on an image-receiving material, and energy is applied to the thermal transfer printing sheet by a heating device such as a thermal head according to image information so as to transfer the dye contained in the sublimable dye layer of the thermal transfer printing sheet to the image-receiving material, thereby producing an image on the image-receiving material.
The hot-melt transfer printing process is as follows: a thermal transfer printing sheet comprising a substrate sheet, and a hot-melt ink layer provided thereon, containing a coloring agent such as a pigment, and a vehicle such as a wax is superposed on an image-receiving material, and energy is applied to the thermal transfer printing sheet by a heating device such as a thermal head according to image information so as to transfer the softened hot-melt ink layer to the image-receiving material, thereby producing an image on the image-receiving S material.
The above-described heat-sensitive sublimation transfer printing process is advantageous in that it can produce an image excellent in gradation because the amount of the dye to be transferred to the image-receiving material can be controlled in each dot by changing the amount of energy applied to the thermal transfer printing sheet. However, monotone images such as letters or symbols produced by this process are poor in sharpness, so that this printing process is not suitable for the recording of is OCR letters, bar codes or the like which will be optically read. The hot-melt transfer printing process is advantageous in that it can readily produce monotone images such as letters or symbols. However, it is difficult to produce, by this printing process, an image with gradation such as a photograph of a person's face.
In order to solve the aforementioned problems in the thermal transfer printing processes, a complex-type thermal transfer printing sheet comprising a sublimable dye layer and a hot-melt ink layer which are alternately provided on one surface of a continuous substrate sheet is used for conducting transfer printing. The thermal transfer printing sheet 71 shown in FIG. 12 is one example of such a thermal transfer printing sheet. This thermal transfer printing sheet comprises a sublimable dye layer 72 composed of sections (72Y, 72M, 72C) of three colors of yellow (Y), magenta (M) and cyan (C), and a hot-melt ink layer composed of a black ink layer 73 (BK) and a protective layer 74 (OP). In this thermal transfer printing sheet, the sublimable dye layer and the hot-melt ink layer are sequentially provided on one surface of the substrate sheet in the order of Y-M-C-BK-OP, and this unit Y-M-C-BK-OP is provided repeatedly.
The above thermal transfer printing sheet is used, for example, in the following manner: after an image with gradation is formed by using the sublimable dye layer 72 (72Y, 72M, 72C), and letters etc. are formed by using the black ink layer 73 (BK) on the surface of an image-receiving material, the protective layer 74 (OP) is transferred to the entire surface of the image and letters etc. formed (the first repeating unit Y-M-C-BK-OP is used for these steps); the image-receiving material is then turned over, and letters etc. are formed by using the black ink layer 73 (BK) on the back of the image-receiving material (the second repeating unit Y-M-C-BK-OP is used for this step). Therefore, among the second repeating unit Y-M-C-BK-OP, only the black ink layer 73 (BK) is used, and the sublimable dye layer 72 (72Y, 72M, 72C) and the protective layer 74 (OP) are not used at all. The second repeating unit in which only the black ink layer 73 (BK) is used cannot be used to form an image on a new image-receiving material any more, so that the third and fourth repeating units have to be used for this purpose. This means that the sublimable dye layer (72Y, 72M, 72C) and the protective layer 74 (OP) are left unused in every other repeating unit Y-M-C-BK-OP provided on this thermal transfer printing sheet. This thermal transfer printing sheet is thus poor in the efficiency of double-side transfer printing, and it is difficult to decrease the running cost.
The present invention has been accomplished in the light of the aforementioned circumstances. An object of the present invention is therefore to provide a thermal transfer printing sheet useful for efficiently transferring images, letters, symbols or the like on both surfaces of an image-receiving material by a thermal transfer printing process, and a process of double-side transfer printing using the same.